1. Field of the Invention
The present invention relates generally to nuclear fuel pellet inspection and, more particularly, is concerned with an apparatus for inspecting fuel pellets for surface defects that employs a pellet slide and inspection assembly providing improved inspecting and handling of fuel pellets.
2. Description of the Prior Art
Nuclear reactors include fuel assemblies which contain pellets of fissionable material as their basic fuel element. In one exemplary embodiment, a pellet ideally takes the form of a right cylinder with slightly concave or dished opposite ends. For incorporation into fuel assemblies, a number of pellets are stacked end to end in a fuel rod cladding tube which, like the pellets, is usually of circular cross-section. Then, a given number of fuel rods are grouped together in a fuel assembly.
It is essential that all pellets used in the fuel assembly be free of circumferential defects, such as cracks and chips, in order to achieve desired stacking of the pellets within the fuel rod tube as well as uniform heat transfer between the stacked pellets and cladding tube and uniform consumption of the pellets during operation of the reactor core. Consequently, an important step in the manufacture of the nuclear fuel pellet is the inspection of its surface to ascertain whether there are any deflects present.
Exemplary prior art systems for inspecting and classifying nuclear fuel pellets are disclosed in U.S. Patent Nos. to Jones (U.S. Pat. Nos. 3,221,152; 3,272,332; and 3,282,116), Ryden, Jr. (U.S. Pat. No. 4,037,103), Marmo (U.S. Pat. No. 4,193,502), and Wilks et al (U.S. Pat. No. 4,349,112). While the inspection systems of these prior art patents appear to achieve their objectives under the range of operating conditions for which they were designed, none of these systems appear to be adapted to perform inspection of a pellet for circumferential or surface defects.
One commercial inspection apparatus, sold under the registered trademark, Inspector General, by Cochlea Corporation of San Jose, Calif., for inspecting and sorting small parts uses non-contact, three-dimensional ultrasonic vision to verify the identity, shape, defects, orientation, and sequence of parts. Its overall objective is to acoustically detect and cull out parts with shape defects. For a detailed understanding of this Cochlea Corporation inspection apparatus, attention is directed to a publication entitled "Inspector General User's Guide" dated Aug. 1987 and to U.S. Patent Nos. to Buckley et al (U.S. Pat. Nos. 4,557,386; 4,576,286; and 4,690,284) and Pinyan et al (U.S. Pat. No. 4,677,852) assigned to Cochlea Corporation.
Basically, inspection by this apparatus is accomplished while the parts are in transit. The parts to be inspected are fed down a chute from a vibratory bowl feeder by a singulation device. As each part is in transmit, it is exposed to 40 kHz sound waves emanating from suitable positioned emitters. The waves bounce off the part, and the reflections are then picked up by an array of transducers or sensors. Analysis of reflected sound waves gives each part an unique "acoustic signature", which is compared to a previously "learned" good part signature. Acceptance or rejection is based on the comparison. This acoustically-based system is said to outperform and be more versatile than more traditional vision, laser, tactile and proximity-sensing techniques. However, in its approach to parts handling and positioning, this "off-the-shelf" inspection apparatus is designed for general purpose parts inspection and sorting and thus cannot be used directly without modification, to inspect nuclear fuel pellets.
Certain improvements were made to this "off-the-shelf" inspection apparatus in U.S. Pat. No. 4,894,201 to Ahmed, which is assigned to the same assignee as the subject invention. These improvements adapt the apparatus for inspection of fuel pellets for surface defects, such as chips and cracks, by substituting a single pellet feeder, a pellet guide chute assembly, and a pellet discharge conveyor. The pellet guide chute assembly is inclined and extends through the inspection chamber. In sliding down the guide chute assembly, the pellets move along an inclined straight, or linear, path one at a time through the inspection chamber. Slots are provided in the chute assembly at the inspection chamber for receiving an ultrasonic inspection head. The inspection head mounts acoustical energy transmitting and receiving transducers which transmit acoustical energy into and received it from the inspection chamber for propagating such energy to and from the pellet as the pellet slid under the influence of gravity down the inclined chute assembly. The pellet position along the inclined chute assembly is sensed by light transmitting and receiving devices. Light is transmitted across the inspection chamber through openings in the chute assembly and thus across the path of movement of the pellets.
A drawback of the improved apparatus of the cited Ahmed patent is that, other than the particular inclination chosen for the chute assembly itself, there is no means of controlling the speed of the pellets as they slid down the inclined straight path defined by the chute assembly. The rapid rate at which pellets are allowed to slide uncontrolled down the straight inclined chute assembly often result in high impacts at the bottom with the pellet discharge conveyor, leading to cracking or chipping of the inspected pellets.
Consequently, a need still exists for additional improvements to better adapt this "off-the-shelf" ultrasonic inspection apparatus for use in inspection of nuclear fuel pellets.